The invention relates to a polyester ionomer composition.
As described in U.S. Pat. No. 6,066,694 to Chisholm et al., the utilization of alkylene aryl polyester copolymers having metal sulfonate units in blends with polycarbonate, polyester carbonates, and polyarylates can result in enhanced properties. The use of a metal sulfonate polyester copolymer modifies the rheology of the blends, especially under low shear where the melt strength is enhanced. Enhanced melt strength is very beneficial in facilitating processing under low shear conditions like blow molding and extrusion, it may also be useful for enhanced thermoformability.
Polyesters are inherently susceptible to hydrolytic degradation. Hence, it is desirable to enhance hydrolytic stability of polyester ionomer compositions.
The rate of hydrolysis of a polyester ionomer may be reduced by converting the carboxylic acid end-groups (COOH) of the polymer, which catalyze ester hydrolysis, to some other moiety that does not catalyze hydrolysis. We have found that modification of a polyester ionomer resin with two different epoxide compounds differing in both the level of epoxide functionality and the nature of substitution on the epoxide ring and catalysts, such as salts of aliphatic carboxylic acids, provides a composition which possesses good hydrolytic stability.
The end capped polyester ionomer composition comprises an alkylene aryl polyester copolymer with metal sulfonate units. The COOH end capping agents comprise a mono- or difunctional cyclic aliphatic epoxide in which the epoxy reactant compound has at least one cyclohexane ring moiety and one or two epoxy groups with at least one of the two epoxy groups being a substituent on a cyclohexane ring and a multifunctional epoxy component possessing at least 3 or more epoxide groups which are not attached to cyclohexane ring moieties.
The end caps produced by reaction of carboxylic acid end-groups of the polyester ionomer with epoxides comprise aliphatic ester groups.
To form the aliphatic ester end capping, an effective amount of the epoxy compounds for forming the end-groups and an effective amount of a catalyst compound are desirable with the polyester ionomer composition.
The combination of a mono- or difunctional cyclic aliphatic epoxide in which at least one epoxide group is a substituent on a cyclohexane ring and a multifunctional epoxide in which the epoxide groups are not attached to cyclohexane ring moieties provides improved hydrolytic stability without substantially reducing the flowability of the composition.
The higher reactivity of the mono- or difunctional cyclic aliphatic epoxide component as compared to the multifunctional epoxy component allows for selective capping of the COOH end-groups of the polyester ionomer by the mono- or difunctional cyclic aliphatic epoxy component during melt processing of the composition and results in the multifunctional epoxy component being largely unreacted during melt processing of the composition. Selective capping by the mono- or difunctional epoxide component during melt processing of the material desirably allows for capping of COOH end-groups without substantial viscosity build and makes available epoxide groups from the multifunctional epoxy component for reaction with polyester ionomer acid groups generated from hydrolysis reactions during the service life of a molded part. Reaction of polyester ionomer acid end-groups during the service life by the multifunctional epoxy component serves to maintain mechanical properties since the overall molecular weight of the composition is more adequately maintained by reaction of the polyester ionomer acid end-groups with the multifunctional epoxy component as compared to reaction of polyester ionomer acid end-groups with the mono- or difunctional epoxy component.
The higher reactivity of the cyclic aliphatic epoxy component relative to the multifunctional epoxy component is due to the greater epoxide ring strain resulting from the epoxide being a substituent on a cyclohexane ring.